Loud "CLUNK" from rear over sharp bumps

[socratic]

I've been noticing more and more a lound "Clunk" from the rear of the car in the neigborhood (best guess) of the right rear shock tower whenever I go over a sharp bump. I think it's getting worse slowly. I also think I hear a fainter version of this from the left side, but not 100% sure.

Would anyone hazard a guess as to what this might be?

I'm hoping its something simple like a the nut on top of a shock tower working its way loose. Even if it is this "nut", I'm not sure how to tighten it.

Could it be something serious like an engine mount?

The car drives normally and the engine seems to be running well (I'm not sure what a broken engine mount might sound like).

I was poking around and when I open the top so that the metal cover that raises 45 degrees to let the roof down [gee, what's this called?] is open, I see two big bolts under it in the same channel as the rear center stop light. On top of these two big bolts are two others things that look like caps. The "caps" are about the same size as the bolts under them. The right cap was slightly loose (only finger tight). Are the big bolts under the "caps" by any chance the the things that I can tighten to lock down whatever it is that is making that "CLUNK"? Are the "caps" only supposed to be finger-tight?